1. Field of the Invention
The present invention relates to a spot welding system including a robot. The present invention also relates to a method for adjusting a closing speed of a welding gun, which can be performed in the spot welding system.
2. Description of the Related Art
Spot welding is known as a type of resistance welding technique, in which a plurality of objective workpieces to be welded, are placed on each other and held between a pair of electrodes (referred to as “an electrode pair” in this application) of a welding gun, and a predetermined current is applied to the held points of the objective workpieces during a predetermined period while applying a predetermined pressurizing force to the held points, so as to obtain a weld of a desired quality. The welding gun is typically configured such that a movable-side electrode is actuated to open or close with respect to a stationary-side electrode, and in recent years, a servo-motor tends to be used frequently as a driving source for the movable electrode. Further, the spot welding system applied to an automobile assembly process, etc., has typically adopted a robot system configuration in which the welding gun as a working tool (i.e., an end effector) is attached to a distal end of an arm structure (i.e., a manipulator) of an industrial robot having an articulated or other configuration (simply referred to as a “robot” in this application), so as to quickly and accurately weld a plurality of junction points on the objective workpieces while following their various positions and orientations. Alternatively, when the objective workpieces are relatively small, another system configuration has also been adopted, in which a robot grips objective workpieces by a hand and operates variously with respect to a welding gun disposed in a stationary state, so as to sequentially locate a plurality of junction points at a predetermined welding position.
When attempting to hold objective workpieces between an electrode pair of a welding gun by a target pressurizing force, it is important to appropriately adjust a moving speed (referred to as a “closing speed” in this application) of a movable electrode in a direction of a closing motion of the electrode pair, in terms of improving quality of a weld point and reducing a cycle time of a welding process. For example, if the closing speed of the movable electrode is too fast, an overshoot of a pressurizing force applied to the objective workpieces may become too large and thus a welding current may start to be applied in a state where the pressurizing force is not yet sufficiently stabilized at the target value, and, as a result, a welding quality may be degraded. Further, in this case, the welding gun and the objective workpieces may be damaged. On the other hand, if the closing speed of the movable electrode is too slow, the time required for the pressurizing force applied to the objective workpieces to reach the target value may become longer, and thus a welding current may start to be applied in a state where the pressurizing force is insufficient, and also welding quality may be degraded. In either case, a certain level of welding quality can be ensured by configuring the system in such a manner that the welding current starts to be applied after the pressurizing force is stabilized at the target value, but a problem of increasing cycle time may arise.
The above-described adjustment of the closing speed of the electrode pair in the welding gun is conventionally performed for respective welding guns manually by an operator. More specifically, while using a pressure sensor for sensing the pressurizing force between the electrodes, an operator monitors an output waveform of the pressure sensor when actually actuating the electrode pair to close, and manually and finely tunes the closing speed through a trial and error so that the overshoot with respect to the target pressurizing force can fall within an tolerable range. However, due to the fact that the welding gun, even of an identical type, typically has an individual structural difference in, for example, friction of moving parts, rigidity of components, etc., and a single welding gun is subjected to a deterioration with age in characteristics such as an electrode wear, etc., and therefore, in the above-described closing-speed adjusting technique by the manual work, there are problems in that excessive burdens are imposed on the operator and significant time is required to start up the system.
In this connection, in a welding gun with an electrode pair actuated by a servo-motor as a drive source, a pressurizing-force control method capable of appropriately control a pressurizing force has been conventionally proposed, in which, in addition to the control of a motor current, a protruding amount of the movable electrode (i.e., a pushing amount relative to the stationary electrode) is automatically corrected based on an actual pressurizing force generated between the electrodes (see Japanese Unexamined Patent Publication (Kokai) No. 6-312273 (JP-A-6-312273)). In the pressurizing force control method as set forth in JP-A-6-312273, the movable electrode is decelerated, during a closing motion of the electrode pair, from a position sufficiently short of a contact position relative to the objective workpieces so as to contact with the objective workpieces at a low speed, and thereby the pressurizing force is controlled to ensure that “the contact is always performed from an undershoot direction”. Then, by using a detection value of the motor current, the actual pressurizing force generated between the electrode pair is monitored, and if the actual pressurizing force does not reach a set value (or a target value) even after a predetermined time has elapsed after the start of monitoring, the protruding amount (or the target position) of the movable electrode is corrected to compensate the deviation of the pressurizing force. According to this technique, collision between the electrode pair and the objective workpieces can be reduced, and the appropriate pressurizing force can be ensured, as occasion demands correspondingly to the wear of the electrodes.
The pressurizing force control method for the welding gun, as set forth in JP-A-6-312273, has a configuration wherein the pressurizing force is controlled in such a manner as to always exhibit an undershoot response pattern, by making the movable electrode contact with the objective workpieces at a low speed, and therefore, time required to stabilize the pressurizing force tends to increase and, as a result, the welding quality may be degraded or the welding cycle time may be increased as described above. The above method also has a configuration wherein the protruding amount (or the target position) of the movable electrode is corrected based on a judgment about convergence (i.e., a judgement concerning whether the actual pressurizing force reaches the set value ox not) performed after a predetermined time has elapsed from the start of the monitoring of the actual pressurizing force (i.e., in a steady state), and therefore, time required to control the pressurizing force may also increase. Further, in the correction of the protruding amount of the movable electrode, a position command is computed while considering the deflection of components provided for the stationary electrode, and therefore, it is required to prepare data of respective welding guns for accommodating the individual structural difference thereof, which makes a system starting-up work complicated.